The invention is based on a discharge vessel having electrodes and a fill which comprises at least one noble gas and a halogen, which, when the lamp is operating, form together an excimer, the discharge vessel being manufactured from quartz glass, and may be used in an associated discharge lamp. This fill is referred to as excimer fill, which in particular includes aggressive fill components, such as above all F or Cl.
U.S. Pat. No. 5,898,265 has disclosed the use of protective layers in fluorescent lamps, which are applied between the discharge vessel and the phosphor layer, protecting the lamps from becoming gray as a result of Hg bonding to the lamp glass. The layers are predominantly applied by using suspensions of crystalline particles, in particular of Al2O3, in the nanometer range. AlonC is generally used, which is crystalline, extremely finely dispersed xcex3-Al2O3 with a mean grain diameter of approx. 10 nm. Mixed layers comprising xcex3-Al2O3 and xcex1-Al2O3 are also described, providing further advantages for the same application, cf. U.S. Pat. No. 5,514,932. The layers used are exclusively crystalline, are applied via a suspension and have layer thicknesses in the micrometer range.
The same principle is also used for Hg UV radiators. CH-A 672 380 describes a diffusion barrier layer comprising oxide of Al Hf, La, Th for Hg in UV radiators which use a fill comprising Hg (1 mbar) and Ar (0.7 mbar) and the discharge vessel of which, which is made from quartz glass, is subject to little load: in operation, the wall temperature is approximately 200xc2x0 C. In this case too, the blackening of the bulb is to be reduced. Layer thicknesses of the barrier layer of from 10 to 1000 nm are used. The layers are applied by means of dip coating with the aid of organic or inorganic precursors. The drainage rate selected for the coating solution is 50-80 cm/min., from which a high layer thickness can be inferred. In the case of Al2O3, the heat treatment took place at 600xc2x0 C. For HfO2, an improvement in transmission over the service life (3000 hours) was detected.
U.S. Pat. No. 5,270,615 describes protective layers which in metal halide high-pressure discharge lamps are to act as a diffusion barrier preventing the metal halides from attacking the quartz glass discharge vessel. Multiple layers comprising oxides of Ta or Nb and comprising Al2O3 are applied in order to gradually match the thermal expansion.
It is an object of the present invention to provide a discharge vessel being manufactured from quartz glass, and having an excimer fill, the vessel having electrodes and a fill which comprises at least one noble gas and a halogen, which, when the lamp is operating, form together an excimer, which is distinguished by a high service life, even though it uses highly aggressive filling substances. a further object is to provide an associated lamp with such a vessel.
This object is achieved by the inner wall of the discharge vessel being completely covered with a passivation layer of an oxide formed from at least one of the metals Al, Hf, Y or Sc, the layer having an amorphous structure and its total layer thickness being 20 to 500 nm. Particularly advantageous configurations are to be found in the dependent claims.
Hitherto, the halogen fluorine, as a highly aggressive filling component, was not useable in glass vessels. This also applies to a lesser extent to chlorine.
Quartz glass as a preferred material for the discharge vessel of lamps is subject to limits on account of reactions with aggressive filling components, such as fluorine, chlorine and, to a lesser degree, for bromine, iodine. Therefore, hitherto long-life excimer lamps were only possible with the filling system Xe2+, which emits at 172 nm. New filling systems for excimer lamps (e.g. XeCl: xcex=308 nm; KrCl: xcex=222 nm; XeF: xcex=351 nm; KrF: xcex=248 nm; ArF: xcex=193 nm) are of considerable interest on account of the further discrete UV wavelengths which are used for various technical processes. Existing chloride-containing systems are not optimally attuned.
A typical cold filling pressure for the entire fill of these excimer radiators is 20 to 400 mbar, the partial pressure of the halogen compound typically forming between 1 and 10% of the total filling pressure.
Fluorine, as a particularly aggressive fill medium, has not hitherto been used in excimer radiators in quartz glass vessels. The fluorine atoms and ions which are formed in the discharge react immediately with the quartz glass, to form silicon fluoride and oxygen. This consumption of fluorine means that less fluorine is available for the discharge, fewer noble-gas excimers are able to form and within seconds it is no longer possible to detect the specific excimer radiation (at 351 nm for XeF*). In order nevertheless to allow discharge lamps to be operated with fill components of this type, it is necessary to discover a cladding material which withstands such an attack. The same applies to a lesser extent to chlorine.
The use of polycrystalline aluminum oxide (PCA) for the discharge vessel, which naturally behaves passively with respect to the fluorine-containing fill, inherently represents one solution. However, the use of a ceramic of this type entails various drawbacks. For example, the vacuum-tight closure and the shaping are very much more complex and expensive than with quartz glass.
Passivation of the quartz glass with an amorphous, transparent Al2O3 protective layer which is from to 20 to 500 nm (preferably at most 200 nm) thick allows the well-controlled quartz glass technology to be retained. Moreover, the optical advantages of quartz glass over ceramic can be utilized. The passivation has to take place over the entire surface of the discharge vessel which interacts with the discharge.
It was not hitherto possible to make commercial use of excimer lamps with fluoride components having a discharge vessel made from quartz glass. The discrete UV wavelengths which in particular fluoride-containing radiators emit were therefore not hitherto available for excimer lamps.
Now, an essential feature is that the layer is amorphous. This amorphous layer leads to an enormous increase in the service life of the lamp, which for the first time ever allows industrial utilization of fluorine excimers. The service life is at least 100 hours and, depending on the particular design, may even exceed 1000 hours. On the other hand, amazingly, it was impossible to detect any barrier action with crystalline layers made from the same material.
The application of suitable layers comprising amorphous Al2O3 results in passivation of the surface of the quartz glass vessel with respect to species which occur in the plasma. In addition to Al2O3, oxides of Hf, Y or Sc or mixed oxides thereof have also proven to be suitable coating materials.
A basic demand imposed on the layer is that the inner surface of the discharge vessel be completely covered, in order to prevent any reactions whatsoever between the quartz glass and the fill. Accordingly, the layer must not have any cracks or open pores. To ensure that is achieved with even greater reliability, it is recommended to apply a second layer (or further layers) of the same material.
The layer is produced using the sol-gel process or alternatively using a CVD process. Details of a CVD process which is suitable for internal coatings are to be found, for example, in the following article: Segner, J.: Plasma Impulse Chemical Vapor Deposition. In: Thin films for optical systems ed. by F. R. Flory, New York (inter alia Dekker, 1995, pp. 203-229).
With the aid of the sol-gel process, it is possible without major outlay, using modified dip coating, to completely coat the inner surface of the discharge vessel. The precursor used is an organic sol based on metal alkoxide, in particular aluminum alkoxide. The rate at which the sol drains out determines the layer thickness and the quality of the layer and must therefore be controllable. To produce crack-free layers, drainage rates in the range from 4-10 cm/min. are preferred. At higher rates, there is an increased risk of cracks forming. With the sol technique, a maximum of approximately 20 cm/min. can be tolerated, but the maximum value is dependent on the sol which is used. The coating operation is followed by a drying operation which causes the wet film to be transformed into a gel film. This gel film is converted into metal oxide (Al2O3) by a heat treatment in a furnace with dissociation of organic components. It has been found that only layers which have been heat-treated at temperatures of 400 to 850xc2x0 C., preferably 500-800xc2x0 C., have a barrier action with respect to fluorine components and a considerable improvement in the barrier action with respect to other halogens. X-ray tests have demonstrated that the layers (or powders) which have been treated at these temperatures were X-ray amorphous. Crystalline transition aluminum oxides (xcex3, xcex4 and xcex8 phases) are only formed at temperatures over 850xc2x0 C., and the thermodynamically stable xcex1-Al2O3 is only formed at above approximately 1100xc2x0 C. All the layers in which these crystalline phases were detected did not have any barrier effect with respect to the fluorine-containing fill and only a weak barrier effect with respect to other halogens.
The Al2O3 layers which were treated at temperatures of 400 to 850xc2x0 C., preferably at 500-800xc2x0 C., appear to be structure free. The particles in the layer are typically of nanometer size. The layers are crack-free and pore-free. The total layer thickness is approximately 20 to 500 nm.
In the case of individual layers, the layer thickness is preferably between 50 and 100 nm.
Layer systems comprising two to five layers are advantageous in order to compensate for any coating flaws and to minimize defects. Often even a double coating is sufficient to hermetically passivate the surface.